[GHC] #10252: Abstract newtype in hs-boot

[GHC]

#10252: Abstract newtype in hs-boot
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        Reporter:  ezyang            |                   Owner:
            Type:  feature request   |                  Status:  closed
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.11
  checker)                           |                Keywords:  backpack
      Resolution:  worksforme        |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by goldfire):

 This scares me. `data` and `newtype` are different! After a quick ponder,
 I don't see damage that can be caused by abusing this.

 But, if we ever had representational type families (where `type instance
 RepF Int = Bool` would be used to reduce `RepF Age` to `Bool`), this would
 be a disaster. I can't seem to find a ticket requesting representational
 matching for type families (or classes, or data families), but the idea
 has come up. (See some wonderings in this direction in #9112, for
 example.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10252#comment:2>
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